The present invention relates to laser etching of a substrate in liquid. Specifically, the invention relates to etching of ceramic, semiconducting and metallic substrates by means of laser induced sonic cavitation. The invention has particular applicability to the machining of slots, rails and grooves in Al.sub.2 O.sub.3 --TiC ceramic, and ferrites in magnetic head sliders.
Laser processing, particularly deposition and etching of a variety of materials, including metals, ceramics, and polymers, which are of interest for electronic device fabrication and for packaging applications, have been demonstrated over the last decade. However, when practicing laser etching or machining, in air or in liquids at high fluence (power density) it is difficult to avoid the formation of a recast layer and the appearance of cracks in the etched substrate. The presence of such "frozen lava" and accompanying defects significantly hinders some functions of the ceramic substrate and therefore limit the use of laser processing with ceramic materials.
A known method of laser etching is micromachining using a combination of laser energy and a chemically reacting liquid etchant in which the function of the laser is to locally heat the substrate and thus locally accelerate the chemical reaction. The importance of the submerged liquid laser enhanced process is demonstrated by its ability to etch brittle ceramic materials, semiconductors and metals with laser energy which would otherwise crack and oxidize the same material when etched in air and would locally alter its properties. Commonly used etchants include aqueous alkaline or acid solutions of KOH, NaOH, HNO.sub.3, H.sub.3 PO.sub.4, HCL and the like. The etch rates vary depending upon the particular material to be etched, type and concentration of chemical solution used, laser wave length and fluence (power density). While alkaline hydroxides or acids in solution are advantageous due to their flexibility for etching, at the same time the very same solutions are disadvantageous because even at room temperature they tend to react with the substrate being etched. Additionally, if the substrate is used as a carrier of metallic devices, components or conductors, such as for instance thin film heads, they may react with the materials of which the device is constructed and destroy it.
In practice, the laser beam is focused to a micron-size spot which is used for etching the material by direct writing. Etching of various materials has been reported. For example, the use of either KOH or NaOH to laser-machine various materials, including PbTiO.sub.3, Al.sub.2 O.sub.3 --TiC and Mn.sub.0.6 Zn.sub.0.4 Fe.sub.2.3 O.sub.4 has been reported in articles by F. Bunkin et al, entitled "Si Etching Affected by IR Laser Irradiation", in Appl. Phys., vol. A37, 117 (1985); T. Shiosaki, et al, entitled "Laser Micromaching of a Modified PbTiO.sub.3 Ceramics in KOH Water Solution", in Jpn. J. Appl. Phys., Suppl. 22-2, 109 (1983); K. Koyabu et al, entitled "Laser-Assisted Etching for Al.sub.2 O.sub.3 /TiC Ceramics Using Nd:YAG Laser and KOH Solution", in J. Jpn. Soc. Precis. Eng., vol. 53, 1027 (1987) and E.K. Yung, et al, entitled "Laser-Assisted Etching of Manganese-Zinc-Ferrite", in J. Electrochem. Soc., vol. 136, 665 (1989). In each of these articles, CW lasers are used with various temperature distributions depending upon the wavelength, absorption depth and material thermal properties. Etching of silicon and Al.sub.2 O.sub.3 --TiC ceramic using concentrated KOH solution and a CW argon laser is described in an article by von Gutfeld et al entitled "Laser Enhanced Etching in KOH", in Appl. Phys. Lett., Vol. 40, 352 (1982).
As noted above, the use of alkaline hydroxide solutions even at room temperature may demonstrate some varied etching effect on the material to be etched, and the function of the laser is to locally accelerate the reaction and provide local very rapid accelerated etching. If the substrate material has fabricated on it devices which react with the alkaline hydroxide or acid, then even if the substrate is hardly affected by the alkaline hydroxide at room temperature, the devices fabricated on the substrate may be destroyed by the alkaline or acid solution.
Use of a pulsed Nd:YAG laser to machine silicon nitride ceramic in water is described in the article by N. Morita et al, entitled "Pulsed Laser Processing of Ceramics in Water", in Appl. Phys. Lett., vol. 52, 1965 (1988). The article states that the process is used with ceramics which sublimate and that alumina ceramics, which fundamentally have a melting point, cannot be processed without creating a recast layer.
Etching and cutting of alumina ceramic such as Al.sub.2 O.sub.3 --TiC causes serious problems due to the high melting temperature of the material. Previously, mechanical machining has been the only method available for microscopic patterning of the material. Recently, reactive ion etching has been shown to be useful for etching alumina ceramic. However, the use of reactive ion etching causes difficulty with the metallic film of which the thin film magnetic head is constructed and which is susceptible to corrosion damage due to the presence of reactive species. Another etching technique includes the use of an excimer laser for etching by ablation. The formation of debris and/or recast (surface crust layer) during ablation is unavoidable and may result in problems requiring additional processing steps for debris removal.